Utility power plants are generally operated based on a control scheme designed to regulate the generating unit power output, in order to meet a power demand set-point selected for one or more loads and thereby to maintain overall power balance in the electrical system. To provide generator power regulation, control strategies have been developed which link the automatic operation of a unit's boiler and turbine generator to act in a coordinated fashion and respond to long-term changes in power demand. These control strategies, which have become relatively standard in the power generation industry, generally involve performing a form of frequency control at the turbine level. By monitoring deviations of the turbine generator's rotational frequency, changes in electrical power demand are matched, in a reactionary manner, through a corresponding change in the mechanical power provided by the boiler. Thus, overall power balance in the system is preserved.
Response systems for handling short-term trends in power demand or other unanticipated electrical load transients have also been developed and integrated into some utility power plants. Transients such as a load rejection or switch-off can require an immediate turbine response due to the reduction in load demand. High-pressure and low-pressure bypass systems can be used to reduce electrical output by routing excess steam from the generating unit's boiler to a condenser (bypassing the turbine) until the boiler, with its generally high thermal inertia, has time to respond and lower its mechanical output to match the reduced steam demand of the turbine. If there is no high-pressure or low-pressure bypass system installed, an electromechanical relief valve (ERV) can be used alternatively to release excess steam into the atmosphere in response to pressure buildup, thereby reducing the supply of steam to the turbine to meet demand. However, atmospheric release of steam can lower overall plant efficiency due to the energy waste. For transients that increase power demand, such as load switch-on, a similar bypass system can be used to route additional steam to the turbine, assuming a source of available steam has been incorporated into and is presently enabled within the bypass system, until the boiler has time to catch up its steam output to meet the increased demand.